1. Field of the Invention
This invention relates to a current and/or voltage detector for a distribution system, and, more particularly to a current and/or voltage detector for an aerial distribution line.
2. Description of the Prior Art
In the modern world where even an instantaneous power failure is not allowed, it is required to always monitor the conditions of power transmission and distribution lines. It is well known in the art that the detection of various currents such as a zero-sequence current flowing through a transmission or distribution line is very useful in monitoring the condition of the line. Also, voltages of various parts of the line are beneficial in the monitoring. In order to detect such a current and/or voltage, a distribution line is provided with many detectors. An example of such detectors is a zero-sequence current transformer (hereinafter, referred to as "ZCT") which is installed in a aerial distribution line.
A ZCT used in a three-phase transmission line comprises a ring type core through which all the three wires pass. When a ZCT is installed in an aerial three-phase distribution line, the ZCT is generally disposed in a pole line switch having a relay so that an unbalance in the currents or voltages of each phases can be detected. A conventional line switch is shown in FIG. 7, wherein a three-phase line 71a is connected through a line switch 72 and tension insulators 73 mounted on a pole 74 to another three-phase line 71b. A ZCT and relay (which are not shown) are incorporated in the line switch 72. If a fault occurs in the line 71a or 71b, the ZCT detects the zero-sequence current to actuate the relay, thereby actuating the switch 72 to interrupt the connection between the lines 71a and 71b.
In this configuration of the ZCT, however, the three wires to which the phase voltage is applied are gathered in the window of the ZCT, resulting in that it is very difficult to insulate the wires from each other. Further, the insulation between the three wires deteriorates in the course of service or depending upon the degree of air tightness of the line switch 72, causing an inferior reliability of the ZCT or a serious fault such as a shortcircuit between the lines. This may be supported by the fact that a relatively large number of accidents have been caused in the vicinity of a ZCT. In order to prevent a fault of this kind from arising, it is required to periodically replace the wires in a line switch with new ones.